1. Field of the Invention
The invention is related to the field of magnetic storage systems, and in particular, to a disk drive including a current-perpendicular-to-plane (CPP) read sensor with multiple reference layers.
2. Statement of the Problem
In many magnetic storage systems, a hard disk drive is the most extensively used to store data. The hard disk drive typically includes a hard disk along with an assembly of write and read heads. The assembly of write and read heads is supported by a slider that is mounted on a suspension arm. When the hard disk rotates, an actuator swings the suspension arm to place the slider over selected circular data tracks on the surface of the rotating hard disk. An air flow generated by the rotation of the hard disk causes the slider with an air bearing surface (ABS) to fly on a cushion of air at a particular height over the rotating hard disk. The height depends on the shape of the ABS. As the slider flies on the air bearing, the actuator moves the suspension arm to position the write and read heads over selected data tracks on the surface of the hard disk. The write and read heads thus write data to and read data from, respectively, a recording medium on the rotating hard disk. Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions.
In a reading process, the read head passes over transitions of a data track in the magnetic medium, and magnetic fields emitting from the transitions modulate the resistance of a read sensor in the read head. Changes in the resistance of the read sensor are detected by a sense current passing through the read sensor, and are then converted into voltage changes that generate read signals. The resulting read signals are used to decode data encoded in the transitions of the data track.
In a typical read head, a current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR) read sensor is electrically separated by side oxide layers from longitudinal bias layers in two side regions for preventing a sense current from shunting into the two side regions, but is electrically connected with lower and upper shields for the sense current to flow in a direction perpendicular to the sensor plane. A typical CPP GMR read sensor comprises an electrically conducting spacer layer sandwiched between lower and upper sensor stacks. The spacer layer is typically formed by a nonmagnetic Cu or oxygen-doped Cu (Cu—O) film having a thickness ranging from 1.6 to 4 nanometers. When the sense current flows across the Cu or Cu—O spacer layer, changes in the resistance of the CPP GMR read sensor is detected through a GMR effect. A typical CPP TMR read sensor comprises an electrically insulating barrier layer sandwiched between the lower and upper sensor stacks. The barrier layer is typically formed by a nonmagnetic oxygen-doped Mg (Mg—O) or Mg oxide (MgOX) film having a thickness ranging from 0.4 to 1 nanometers. When the sense current “quantum-jumps” across the Mg—O or MgOX barrier layer, changes in the resistance of the CPP GMR read sensor is detected through a TMR effect.
The lower sensor stack comprises nonmagnetic seed layers, an antiferromagnetic pinning layer, a ferromagnetic keeper layer, a nonmagnetic antiparallel-coupling layer, and a ferromagnetic reference layer. The upper sensor stack comprises ferromagnetic sense (free) layers and a nonmagnetic cap layer. In the lower sensor stack, the keeper layer, the antiparallel-coupling layer, and the reference layer form a flux-closure structure where four fields are induced. First, a unidirectional anisotropy field (HUA) is induced by exchange coupling between the antiferromagnetic pinning layer and the keeper layer. Second, an antiparallel-coupling field (HAPC) is induced by antiparallel coupling between the keeper layer and the reference layer across the antiparallel-coupling layer. Third, a demagnetizing field (HD) is induced by the net magnetization of the keeper layer and the reference layer. Fourth, a ferromagnetic-coupling field (HF) is induced by ferromagnetic coupling between the reference layer and the sense layer across the spacer or barrier layer. To ensure proper sensor operation, HUA and HAPC should be high enough to rigidly pin magnetizations of the keeper layer and the reference layer in opposite transverse directions perpendicular to the ABS, while HD and HF should be small and balance with each other to orient the magnetization of the sense layers in a longitudinal direction parallel to the ABS.
In the flux-closure structure of the CPP TMR read sensor, the Co—Fe keeper layer is selected to ensure high exchange and antiparallel coupling. Its composition is optimized and its magnetic moment is small, so that high HUA and HAPC can be attained. The Co—Fe—B reference layer is selected to ensure a strong TMR effect and mild ferromagnetic coupling. Its B content is high enough for B atoms, which are much smaller than Co and Fe atoms, to occupy interstitial sites of a crystalline structure and thus interfere with the ability of the Co and Fe atoms to crystallize. As a result, an interstitial-type amorphous film with a flat surface is formed, which facilitates the Mg—O or MgOX barrier layer to grow with a preferred <001> crystalline texture on the flat surface, thus increasing a TMR coefficient (ΔRT/RJ) and decreasing HF. Its Co and Fe contents are optimized and its magnetic moment is small, so that a high HAPC can be attained.
The use of the Co—Fe—B reference layer in the prior art generally meets the requirements of high HAPC, low HF, and high ΔRT/RJ. However, it is still desirable to further improve the reference layer for the CPP TMR sensor to operate more robustly.